Q factor, Q stability and gain in active filters

Abstract

This letter sets out a simple theory relating the maximum Q factor of a second-order active-filter section to the stability of that Q factor and the available gain in the active elements used. The theory is shown to apply to both the ‘negative-feedback’ or ‘Rauch’-type filters and ‘positive-feedback’ or ‘Sallen–Key’-type filters and by implication to negative-impedance convertors (n.i.c.s) and the whole family of circuits based on minimum-phase feedback networks.